This invention relates to displacing oil within a subterranean reservoir by injecting steam and steam-foaming surfactant into the reservoir. More particularly, it relates to forming a steam foaming surfactant mixture which is efficient even in the presence of polyvalent metal ions, such as divalent cations.
The present invention provides an improvement for steam drive or steam soak processes such as those described in U.S. Pat. No. 4,086,964 by R. E. Dilgren, G. J. Hirasaki, H. J. Hill, and D. G. Whitten, which describes a steam-channel-expanding steam foam drive process in which a steam-foam-forming mixture containing steam and a steam-foaming surfactant is injected into and displaced through a steam channel extending between injection and production locations, or U.S. Pat. No. 4,393,937 by R. E. Dilgren and K. B. Owens, which describes a cyclic or continuous steam foam drive process for displacing oil within a subterranean reservoir using steam and a steam-foaming surfactant which consists essentially of an olefin sulfonate surfactant. In general, those processes are efficient, but the propagation of the steam foam may be delayed where the reservoir rocks tend to exchange a significant amount of divalent ions for monovalent ions contained in a solution being displaced through the reservoir. The disclosures of those patents are incorporated herein by reference.
In a process for displacing oil within a subterranean reservoir by injecting a mixture of steam and steam-foaming surfactant, an efficient transport of the surfactant through the reservoir is important. The rate at which foam moves through the reservoir cannot exceed the rate at which that surfactant is transported.
Many subterranean reservoirs have an ion-exchange capacity which is sufficient to impede the transport of a steam-foaming surfactant through the reservoir. Such an ion-exchange is due to a mechanism by which monovalent ions in an injected surfactant solution (such as the aqueous phase of a mixture pf steam and steam-foaming surfactant) displace multivalent cations from the clays and the like ion-exchange sites on the reservoir rocks. This exchange results in a buildup of the multivalent cation content in the injected surfactant solution.
Of all the mechanisms affecting the surfactant transport in a steam foam process, ion-exchange is one of the most critical mechanisms. Other mechanisms including partitioning, precipitating, and possibly spreading behavior of the oil and the gas/aqueous interface, are directly related to the multivalent cation concentration of the aqueous liquid phase.
In the presence of oil, a buildup of multivalent ions, such as one due to an ion-exchange, increases the partitioning of the surfactant into the oleic phase. This is due to conversion of normal surfactant micelles to inverted micelles which are soluble in oil. The surfactant transport is thus retarded due to the partitioning of surfactant into bypassed droplets of oil.
U.S. Pat. No. 4,269,271 describes a surfactant-containing emulsion-flooding oil recovery process which is effective for use in salinities higher than about 30,000 ppm total dissolved solids and/or concentrations of divalent ions greater than about 2,000 ppm. That process uses a mixture of at least two surfactants; (a) an alkylpolyalkoxyalkylene or alkylarylpolyalkoxyalkylene sulfonate surfactant and (b) a nonionic surfactant. U.S. Pat. No. 4,465,602 describes a surfactant-containing aqueous flood containing an alkylarylpolyalkoxyalkylene sulfonate surfactant.